Various reconnaissance systems are used to intercept radar signals and decipher some of their critical characteristics and angles of arrival. A microwave intercept receiver may be used for just this purpose. In particular reconnaissance applications in areas such as Electronic Warfare (EW), the receiver is designed to fulfill roles such as radar warning, electronic support measures (ESM), and Electronic Intelligence (ELINT). In most conventional approaches, the intercept receiver is designed to perform two functions. The first function is to measure the signal characteristics of the intercepted signal, and the second is to determine its angle of arrival (AOA) for the purpose of direction finding (DF) and location of the radar source.
With the proliferation of radar systems and the increasing number of radars employing complex waveform modulation, it is difficult to differentiate and sort the intercepted radar signals using just the coarse conventional parameters. Typically these coarse parameters include AOA, carrier frequency, pulse width (PW), pulse repetition interval (PRI), and scan pattern. Since many radars have similar conventional parameters, ambiguity may occur in both the sorting and identification processes.
One type of receiver that may be used to precisely measure the conventional parameters as well as the intrapulse modulation for both sorting and identification purposes is the intrapulse receiver.
However, the use of Low Probability of Intercept (LPI) radars with low peak power has introduced a further requirement for modem intercept receivers, requiring them to have a much higher sensitivity in order to detect these LPI radar signals. Until recently, almost all radars were designed to transmit short duration pulses with a high peak power. This type of signal is easy to detect using relatively simple, traditional EW intercept receivers making the attacker (radar source) vulnerable to either antiradiation missiles or Electronic Counter Measures (ECM). However, by using LPI techniques it is possible to design a LPI radar that is effective against traditional EW intercept receivers. One of the most important LPI techniques is the use of phase or frequency waveform coding to provide transmitting duty cycles approaching one. This technique can result in drastic reductions in peak transmitted power while maintaining the required average power.
Therefore, with an increasing number of radars employing complex waveform modulation in addition to using low-peak power LPI signals, it is required that a modern intercept receiver perform the following three basic functions: a) measure and characterize conventional pulsed radar signals; b) detect and characterize LPI signals; and c) determine the AOA for both conventional pulsed signals and LPI signals. Furthermore, these three functions should be performed on the intercepted signals in a multiple signal environment and on a pulse-by-pulse basis.
A current architecture that accomplishes both signal measurement and accurate AOA determination on conventional pulsed signals is an interferometer. In an interferometer, a number of antenna elements are distributed in a two-dimensional plane and phase comparison between different antenna elements is used to determine the AOA. Microwave phase detectors are typically used for phase comparison. Recently these phase detectors have been replaced by digital measurement techniques. The signal characteristics of the intercepted signals are measured either from the output of one of the interferometer antennas or from a separate antenna. Signal characterization is performed using an intrapulse receiver which is traditionally implemented by analog devices. In this case, a frequency discriminator is used for frequency measurement while a Detector Log Video Amplifier (DLVA) is used for amplitude measurement.
Detection of LPI signals is currently accomplished using a channelized receiver instead of an intrapulse receiver. A channelized receiver is typically implemented using either a band of microwave filters with a detector at the output of each filter. Other receivers may be used, such as a time-integrating acousto-optic spectrum analyzer and compressive receiver. The use of a channelizer will reduce the noise bandwidth in each channel and thus increase the receiver sensitivity for LPI signal detection. Other architectures such as correlators are also suitable for LPI signal detection and AOA determination. These correlators are implemented using analog, optical, or digital technology. However, the AOA determination process is quite different from the interferometer approach and very limited intrapulse information can be extracted.
Thus, some of the limitations of current receiver systems are that use of different receiver technologies results in a more complex system architecture and implementation. Since each receiver usually performs only one specific function, elaborate control and correlation of different receiver outputs are required for arriving at a complete picture of a high-density signal environment. If the correlation is not done precisely, ambiguity or even discrepancy in the signal recognition process may occur. Furthermore, the original signal content from each antenna is not preserved in the detection process and thus, in general, cannot be combined with the same signal appearing from other channels for enhancing the overall signal-to-noise ratio (SNR).
It is an object of the present invention to obviate or mitigate some of the above disadvantages.